A bridge circuit having a half-bridge can be used for controlling an electrical consumer such as an electric motor or an electric heater, for instance. The half-bridge encompasses a first switching device for connecting a terminal to a first potential, and a second switching device for connecting the terminal to a second potential. The consumer is operated between the terminal and an appropriate potential. This potential may be fixedly selected or be controlled with the aid of a further half-bridge. The two half-bridges may provide the consumer with mutually complementary potentials, so that a current direction is controllable by the consumer through the actuation of the half-bridges.
The switching devices of the half-bridge must be switched in such a way that both switching devices are not closed at the same time. Otherwise a high short-circuit current would flow through the circuit elements, which could damage or destroy the circuit elements. It should be noted here that if a switching device is implemented as a field-effect transistor, this switching device may let current pass in one direction even when deactivated. To avoid the short-circuit current through the switching devices, a predefined dead time during which none of the switching devices is closed is therefore usually interposed between the opening of one of the switching devices and the closing of another switching device. As a rule, this dead time is determined on the basis of worst-case scenario calculations plus a safety margin.
The longer the dead time, the greater a power loss may be at the half-bridge. If the switching device is open, then the power loss is the product from the flowing current and the voltage dropping across the switching device. In case of a field-effect transistor, the current may lie in a range of 10 A and the dropping diode voltage at approximately one volt. During a dead time of approximately one microsecond, the power loss thus amounts to approximately 10 μJ. In the closed state, the power loss of the switching device is the product from the square of the flowing current and the forward resistance, the latter possibly being in the range of approximately 6 mΩ in a field-effect transistor. In a pulse width modulation having a period duration of 50 μs, the power loss outside the dead time thus amounts to approximately 30 μJ. Although the dead time represents only 2% of the period duration, it consequently has a share of approximately 30% in the power loss.